He-atom scattering study of the temperature-dependent charge-density-wave surface-structure and lattice-dynamics of 2h-tase2(001)

Elastic and inelastic He-atom scattering has been used to measure the surface structure and surface dynamics of the layered transition-metal dichalcogenide 2H-TaSe2(001) crystal. The results cover temperatures from 60 to 140 K. Below T0=122 K an incommensurate charge-density wave (CDW) is formed, which becomes commensurate below approximately 90 K. The measured intensities of the CDW diffraction peaks continuously increase with decreasing temperature below 122 K. From the diffraction intensities, the temperature-dependent amplitude of the surface potential corrugation is determined. The corrugation amplitude is used as an order parameter and from its temperature dependence, on cooling, a critical exponent of =0.33 is extracted. Time-of-flight spectra are used to determine the surface-phonon dispersion curves. Although the spectra are nearly the same at 60 and 140 K, a softening in the Rayleigh mode is observed for intermediate temperatures (around 110 K) at Q=0.53 AI-1, which is near the middle of the Brillouin zone. The difference between the bulk and the surface dynamics is interpreted through the use of the dispersive linear-chain model.